digsig: build dependency fix

Fix build errors by adding Kconfig dependency on KEYS.
CRYPTO dependency removed.

  CC      security/integrity/digsig.o
security/integrity/digsig.c: In function ?integrity_digsig_verify?:
security/integrity/digsig.c:38:4: error: implicit declaration of function ?request_key?
security/integrity/digsig.c:38:17: error: ?key_type_keyring? undeclared (first use in this function)
security/integrity/digsig.c:38:17: note: each undeclared identifier is reported only once for each function it appears in
make[2]: *** [security/integrity/digsig.o] Error 1

Reported-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Dmitry Kasatkin <dmitry.kasatkin@intel.com>
Signed-off-by: James Morris <jmorris@namei.org>